Aerial photo shows a site where the final phase of the Dakota Access Pipeline took place last month with boring equipment routing the pipeline underground and across Lake Oahe. (Tom Stromme/AP)

The Dakota Access pipeline developer said it has placed oil in the pipeline under a Missouri River reservoir in North Dakota and that it's preparing to put the line into service.

Dallas-based Energy Transfer Partners made the announcement Monday in a brief court filing with the U.S. District Court for the District of Columbia. The announcement marks a significant development in the long battle over the project that will move North Dakota oil 2,000 miles through South Dakota and Iowa to a shipping point in Illinois. The pipeline is three months behind schedule due to large protests and the objections of two American Indian tribes who say it threatens their water supply and cultural sites.

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ETP spokeswoman Vicki Granado said in an email to The Associated Press that the line will deliver oil to Patoka, Illinois, within a few weeks.

"Oil has been placed in the Dakota Access Pipeline underneath Lake Oahe. Dakota Access is currently commissioning the full pipeline and is preparing to place the pipeline into service," the court filing stated.

Despite the announcement, the battle isn't over. The Standing Rock and Cheyenne River Sioux tribes still have an unresolved lawsuit that seeks to stop the project. The Standing Rock chairman did not immediately return a call seeking comment on ETP's announcement.

The tribes argue that a rupture in the section that crosses under Lake Oahe would threaten their water supply and sacred sites and would prevent them from practicing their religion, which requires clean water.

A couple embrace as opponents of the Dakota Access pipeline leave their main protest camp on Wednesday, Feb. 22, 2017, near Cannon Ball, N.D. (James MacPherson/AP)

The company disputes the tribes' claims and says the $3.8 billion pipeline is safe.

The tribes in December held up the project by successfully pushing the U.S. government for a full environmental study of the Lake Oahe crossing, which is in southern North Dakota. But the Army Corps of Engineers, which manages the Missouri River for the government, rescinded the study and gave the company permission to complete the pipeline at the urging of President Donald Trump shortly after he took office.

There were months of protests against the pipeline, mainly in North Dakota, where opponents set up a camp on Corps land between the Standing Rock Reservation and the pipeline route. At times it housed thousands of people, many of whom clashed with police, who made about 750 arrests between August and February. The on-the-ground protests waned after the Corps ordered the shutdown of the camp in February in advance of the spring flooding season.

While the protests have abated, opposition has not. The company on March 20 reported "recent coordinated physical attacks" on the pipeline without offering details. Authorities in South Dakota and Iowa confirmed that someone apparently used a torch to burn a hole through empty sections of the pipeline at aboveground shut-off valve sites.

North Dakota has become the second-biggest oil producer in the U.S. in the past decade, trailing only Texas. The state stands to gain more than $110 million annually in tax revenue with oil flowing through the pipe, according to an analysis by The Associated Press.